1. Field of the Invention
The present invention relates generally to extrusion blow molding machines and, more particularly, to an electromechanical drive assembly for the programming and purging actuators associated with an accumulator head in an extrusion blow molding machine.
2. Description of the Related Art
Containers and other hollow articles are frequently formed by blow molding. The blow molding process involves providing a tube of heated and softened plastics material (parison) about which a two-piece mold is closed. Air or another gas is introduced under pressure into the parison to expand it against the walls of the mold, forming the desired hollow article. Such blow molding machines can be of various types, the most common of which are extrusion-blow molding machines and injection-blow molding machines.
In extrusion-blow molding machines, plastics material is heated and softened in an extruder and is conveyed into a die head from which a tubular parison is extruded. The parison can either be continuous, whereby a plurality of molds are sequentially enclosed about successive, axially spaced portions of the parison, or individual parisons can be intermittently extruded and subsequently blown. In the latter instance, the die head includes a chamber, usually called an accumulator, in which the extruded material is accumulated to provide a predetermined volume of material. The accumulated volume of material is then ejected through an annular die at the outlet of the accumulator to form a parison having the desired length, diameter, and wall thickness distribution.
Typically, an accumulator has a purging system to accomplish the function of ejecting the accumulated material by forcing it through the die at the outlet, and a programming system to control the die opening at the outlet for varying the wall thickness of the extruded parison and to close the die while plastic for the next parison is being fed into the accumulator. The speed at which material is ejected by the purging system is often synchronized with the size of the die opening as controlled by the programming system. This synchronization enables the wall thickness to vary in a controlled manner over the length of the parison.
In the prior art, hydraulic cylinders have been used to supply the force required to operate the purging and programming systems. However, these hydraulic actuators do have several inherent disadvantages. The hydraulic oil is subject to dirt and contamination in an industrial environment and requires filtering and maintenance. Further, there is the potential for oil leakage which makes hydraulic systems unsuitable for "clean room" manufacturing environments. Hydraulic cylinders also have limited positioning accuracy and repeatability, and changes in temperature of the hydraulic oil will lead to further variations in performance. Finally, hydraulic systems are not very energy efficient, and therefore require heat exchangers to remove heat from the oil in order to maintain an acceptable oil temperature during machine operation.
Even though the disadvantages of hydraulic systems have long been recognized, electromechanical alternatives have not been developed for the accumulator used in an extrusion blow molding machine. With respect to blow molding machines utilizing continuous parison formation, U.S. Pat. No. 5,338,173 discloses an electrically driven mechanism for parison wall thickness control in a continuous (non-accumulator) system. However, such a mechanism is not suitable for performing wall thickness control in conjunction with an accumulator, since the patented configuration does not allow for introduction of gas through the mandrel, as is required in blow molding systems using an accumulator.
Furthermore, in order for an accumulator to be completely oil-free in operation, there needs to be an alternative drive for the typical hydraulically operated purging system. Since the force required for purging is significant, the friction inherent in mechanical systems will have to be minimized in order to avoid premature wear of the components.